- The mailman list has some 600+ subscribers
- This mailman is part of the CPanel package
- This is a daily (M-F) listserv newsletter. The daily newsletter is only sent out by the moderator.
- The list is fairly stable in size with the usual unsubscriptions and subscriptions, a slight increase each month.
- Mailman had been running fine for months, until Aug/Sep when subscribers started sending messages saying the daily email was not arriving.
- Subscribers NOT receiving their email seem to be consistently the same subscribers. They are from many and varied ISPs including AOL and Earthlink.
- NOT ALL subscribers of any of the ISPs are being rejected. Many subscribers on ALL the ISPs are receiving the daily email just fine.
- Some 250 of the subscribers are now on a Topica listserv. They ALL receive the daily email each day. When the same message is sent to the mailman listserv some of these 250 subscribers do NOT receive the email.
- Emails sent directly to the subscribers, from the same email address that is used to send to the mailman listserv get through just fine, though email sent through mailman does not get to them.

How can this issue be further isolated and resolved?

Are there Cpanel users that are using Mailman with much larger list sizes successfully?